Readout analyzer



J. R. TRAMMELL 3,465,102

READOUT ANALYZER 5 Sheets-Sheet l sept. 2, 1969 Filed Aug. 16, 1966Sept. 2, 1969 .1.R. TRAMMELL READOUT ANALYZER 5 Sheets-Sheet 2 FiledAug. 16, 1966 Sept. 2, 1969 J. R. TRAMMELL 3,465.102

READouT ANALYZER Filed Aug. 1'6, 1966 5 Sheets-Sherri'l :5

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READOUT ANALYZER 5 Sheets-Sheet 4 Filed Aug. 16, 1966 Wnmvws rugs mwNUANXNX Sept. 2, 1969 1. R. TRAMMELL READOUT ANALYZER 5 Sheets-Sheet 5Filed Aug. 16, 1966 Patented Sept. 2, 1969 3,465,102 READOUT ANALYZERJames R. Trammell, Cleveland, Ohio, assignor to the United States ofAmerica as represented by the Secretary of the Air Force Filed Aug. 16,1966, Ser. No. 572,864 Int. Cl. H04m 1/ 74 U.S. Cl. 179-1 3 ClaimsABSTRACT F THE DISCLOSURE A speech analyzer selects a desired voice froma multi- -plicity of voices by examining the harmonic content of thevoices. The desired voice is separated from interfering voices byfiltering the fundamental along with its related harmonics of thedesired voice from the interfering voices.

This invention relates to a readout analyzer and more particularly to amethod and system of segregating certain types of voices and othercomplex sounds by their harmonic content, and separating the desiredsound from interfering sounds for further analysis of the informationcontent by filtering the fundamental along with its related harmonicsfrom the unwanted sounds.

Speech consists of a complex acoustic wave caused by the vibration of apersons vocal chords modulating the stream of air passing through thethroat. This complex wave consists of the fund-amental frequency and alarge number of exact multiple harmonics recurring at the fundamentalfrequency or pitch of the voice. When this complex wave passes throughthe resonant cavities of a persons mouth, certain regions of frequenciesare reinforced by this resonance, which results in formants which arerecognized as vowels and consonants as a person rapidly changes theresonant size of the mouth to form words.

The fundamental pitch of various voices will range from as low as 90cycles per second for a deep male voice to as high as 300 cycles persecond for a high pitched female voice. The pitch variation 0f malevoices ranges from 90 to 140 cycles per second. During this discussionWe will concern ourselves only with the male voice.

A number of recordings were made of a male voice and the energy level ofthe fundamental, and all of the harmonics of various words weremeasured, using a Bruel- Kjear frequency analyzer, and the line spectracharts were made. All of these words were spoken at the same pitch, andconsist of energy at the fundamental and each exact harmonic, thedifference between words being the -amplitude of various harmonics,depending on the resonant size of a persons oral cavities while speakingthat particular syllable or word.

Tape recordings were -made of three diiferent male voices, each voicingthe syllable i as in tip. A closed loop of tape was made of this soundand analyzed with .the Bruel-Kjear analyzer to determine the fundamentalfrequency of each of the three voices. The lowest fundamental frequencywas 95 c.p.s., the highest 120 c.p.s., and one between at 108 c.p.s. Itis quite `apparent that a series of sharply tuned lters, each tuned toone of the harmonies of a particular voice, this voice would predominateby whatever level difference is Iaccomplished by the Width `and shape ofthe filter used. The level by which one voice would predominate willdepend on the difference in fundamental frequency between the desiredvoice and the fundamentals of the unwanted voices. In the case of thethree voices referred to above, this difference amounted to 12 and 13c.p.s. Using a lter of the type employed in the Bruel-Kjear frequencyanalyzer would made possible `a level difference between two tones 12cycles apart of 3 db, so that a readout analyzer of the type describedwould cause a voice, selected by harmonically related filters tuned toits fundamental frequency to predominate the other voices by 3 decibels,provided they had been spoken at the same level. In previous tests itwas determined by recording three voices at the same time that one voicewould become predominate and intelligible if it was recorded at a levelapproximately 3 to 6 decibels above the other two voices.

The limitations of the system of the present invention will bedetermined by the characteristics of the lter used, the difference infundamental frequency of the voices to be separated, and the amount ofdeviation of the fundamental pitch due to inflection of the voice at theend of sentences and during periods of emphasis.

The present invention utilizes a technique of heterodyning thefrequencies with which it is concerned up to some intermediatefrequency, and using crystals to give ya very selective band-pass. Thusa heterodyne system is used for the readout analyzer with a total of atleast 25 crystal lat-tice filters, one for each harmonic and covering afrequency range of c.p.s. to 2500 c.p.s. The frequency chosen for thefilters was 100.1 kc. to 102.5 kc., which would place them 100c.p.sapart and establish this as the pitch of the system. Any recordedvoice could be 'adjusted to this pitch by adjusting :the speed of =atape transport until all harmonics of that voice would simul- Itaneouslypass through the 25 lters, Each of the filters would have to have thenarrowest possible bandwidth, which would still allow a rise and decaytime sufficiently rapid to enable them to follow the syllabic rate ofthe human voice without causing syllables Ito run together.

The object of this invention is to provide a method and system ofselecting all the related harmonics of complex sounds, such as groupconversations, and to separate the voices so one of them willpredominate.

Another object of the present invention is to provide a readout analyzerfor segregating certain types of voices and other complex Isounds bytheir h-armonic content, and separating the desired sound frominterfering sounds for further analysis of the information content of asystem of filtering the fundamental along with its related harmoniesfrom the unwanted sounds.

The various features of novelty which characterize this invention arepointed out with particularity in the claims annexed to `and forming -apart of this specification. For a better understanding of the invention,however, i-ts `advantages and specific objects obtained with its use,reference should be had to the laccompanying drawings and descriptivematter in which is illustrated and described `a preferred embodiment ofthe invention.

In the drawings:

FIGURE 1 shows the block diagram of the basic readout analyzer of thepresent invention;

FIGURE 2 illustrates the line spectrum of two voices saying A as in TAN;

FIGURE 3 shows the passband characteristics of filter 47;

FIGURE 4 is a block diagram of the basic system of speed control for thecapstan motor of tape player 10;

FIGURE 5 shows two reproducer heads separated by a predetermineddistance and their associated tapes that are included in tape player 10to correct for delay and hunting; and

FIGURE 6 shows a complete readout analyzer in block diagram form.

Now referring to FIGURE 1, there is shown tape player 10 which includesa capstan motor. A 100 c.p.s. voice is included in the information onthe tape in tape player 10. This information in electrical signal formis passed through audio amplifier 11 to be fed to balanced modulator 12.Balanced modulator 12 also simultaneously receives a 100 kc. (kilocycle)signal from crystal oscillator 13. Modulator 13 with the 100 kc.insertion signal from crystal oscillator 13 creates upper and lowersidebands for all of the frequencies on the tape.

There are also shown 25 parallel filters which are numbered 16 through40 and are in channels Li6-60, respectively. The inputs thereto aresupplied by the output of balanced modulator 13 by way of line 15 andthe outputs from filters 16-40 are fed to product detector 41 by way ofline 15. Product detector 41 simultaneously receives the 100 kc. signalfrom crystal oscillator 13.

Filters 16, 17-40 have been designed to accept the upper sidebandinformation of the desired signal (100 c.p.s.), that is, 100.1 kc.,100.2 kc. through 102.5 kc. Only this information appears in the outputof the filters and when it is recombined with the carrier signal of 100kc. in product detector 41, the 100 c.p.s. voice appears at the outputthereof and is further processed in audio amplifier 42 which thenprovides an output `at terminal 43. The passband characteristics of arepresentative filter is shown in FIGURE 3.

The complete action may be made clearer if it is assumed that the tapeonly has two voices, both saying A as in TAN (See FIGURE 2 which is aline spectrurn of two voices saying A as in TAN-solid line at afundamental frequency of 100 c.p.s.-dotted line at a fundamentalfrequency of 115 c.p.s.).

Voice No. 1 (solid line) is at a fundamental pitch of 100 c.p.s. and hasharmonic energy at each 100 cycle multiple. Voice No. 2 (dotted line) isat a fundamental pitch of 115 c.p.s. and it therefore has its harmonicenergy at each 115 cycle multiple. The lters will .accept all of theupper sidebands of 100 c.p.s. voice and with a filter passband of i6cycles only the following harmonics of 115 c.p.s. voice will pass: theseventh at 805 cycles, the thirteenth at 1495 cycles, and the twentiethat 2300 cycles. Thus almost 87% of the frequencies necessary forintelligibility of the 115 c.p.s. voice are missing in the output andbecause the 100 c.p.s. voice is allowed to pass with almost noattenuation, it then becomes predominant and easily understood. Therecombination of the reinsertion 100 k.c. signal from oscillator 13 andproduct detector 41 results in only the desired voice being reproduced.

The choice of 100 c.p.s. as a fundamental frequency is arbitrary,because regardless of the actual pitch of the voice, it can be changedto the acceptable frequency of the filter system simply by changing thespeed of the tape recorder. Numerous techniques may be used, but in thisinstance the use of a synchronous motor and a variable frequency 115volt supply provided the best results.

-It is also necessary to correct for another factor in the voice andthat is inflection, because a change of one cycle in the fundamental ofthe voice shifts the seventh harmonic seven cycles and so the seventhand higher harmonies are out of the passband of the filters. Thestringent stability requirements of motor speed correction dictated theuse of crystal type control. The variable frequency signal for aforesaidcapstan motor supply is derived from the heterodyne difference betweentwo crystal oscillators. The crystal oscillators are made variable overa range of cycles for each by the use of voltage Variable capacitors inseries with each crystal. FIGURE 4 is a block diagram of the basicsystem of speed control for the capstan motor of tape player 10. Amanual adjustment is accomplished by the selection of an isolatedvoltage in manual frequency set component 70 which is applieddifferentially to voltage variable capacitors 71 and 72. As the voltageis increased in the positive direction, the action of the voltagevariable capacitor is to increase the frequency of oscillators 73 and 74and conversely for reduction of voltage.

The speed change necessary to compensate for inflection is accomplishedby applying the 200 c.p.s. of the desired voice from tape player 10 to200 c.p.s. lter 77 to discriminator 78 which provides a correctivevoltage to oscillator 74 by way of voltage variable capacitor 72. Thevoltage from the manual control (which is labeled manual frequency setcomponent and the correction network are series added so that if thevoice inflects upward, the capstan motor is made to run slower andcompensates for this change.

A more detailed description now is presented for the system shown inFIGURE 4. The output of mixer amplitier is 10() volts peak to peak overthe frequency range of 50 c.p.s. through 70 c.p.s. which is applied tocapstan motor 76. The frequency change is accomplished by utilizing theheterodyne difference between crystal controlled variable frequencyoscillators 73 and 74. For example, the actual oscillator frequenciesfor a 60 c.p.s. output are 102.280 kc. for oscillator 73 and 103.340 kc.for oscillator 74. Manual frequency set component 70 is a conventionalvoltage source operating differentially, that is if the voltage appliedto voltage variable capacitor 71 is increased; simultaneously, thevoltage applied to voltage variable capacitor 72 is decreased. When thevoltages are changed in this manner, the frequency of oscillator 73increases and oscillator 74 frequency decreases and the differencefrequency is now less than 60 c.p.s. The converse occurs if an increasein output frequency is desired.

Oscillator 74 only is used or the automatic control of the outputfrequency. The DC voltage from discriminator 78 will vary from 0.78 voltto +0.88 volt. The discriminator voltage is applied to voltage variablecapacitor 72 which then serves to alter the frequency of oscillator 74.

The outputs of oscillators 73 and 74 are combined in mixer amplifier 75which is then applied to capstan motor 76 for the purpose of controllingthe speed thereof.

The introduction of the inflection compensation circuitry produced twoadditional problems. All narrow band filters exhibit some delay in thetransit of a signal. The delay is actually a composite of four factors:A delay in the start of the signal; a finite rise time to fullamplitude; storage time; and a nite time for the signal to decay tozero.

For the purpose of this discussion it is expedient to lump the delaysinto rise and decay time and only consider the rise time. The lters usedhave -a total rise time of milliseconds, but because the correctiveelements are only frequency sensitive a usable amplitude of signalappears in the output after 135 msec. Without any compensation, thecorrective signal from the discriminator would change the speed of thetape 135 milliseconds after that portion of the tape had passed thereproduce head which is part of tape player 10. To correct for thisdeliciency, refer to FIGURE 3 where a second reproduce head wasinstalled one inch following the signal head (one inch tape travel at71/2 i.p.s. requires 133 msec.). The first reproduce head 80, the signalhead, now supplies information to the automatic speed control, the 135msec. delay allows this portion of the tape to tarvel to the secondreproduce head 81, the information head, where lthe proper speedcorrection is applied and the voice appears as a monotone at thefrequency which is acceptable to the filter system.

The second problem is hunting The discriminator has zero output voltagewhen the desired signal is in the center of the passband of the filtersystem. It must also have zero output voltage when there is no signal sothat the speed can be accurately set with the manual control. FIGURE 5shows a tape which has blocks of information at different frequenciesrecorded on Track No. 1. Without the anti-hunt provision the followingoccurs:

The first block of information passes reproducer head 80, the c.p.s.signal goes to the discriminator and issues a plus 5 corrective signalwhich speeds up the tape just as that block of information reachesreproducer head 81 so that frequency is now read as 200 c.p.s. and

is acceptable to the filters. But the second block of information is nowunder reproducer head 80 and because the tape speed has increased, thisfrequency is now read as 200 c.p.s. and when it is applied to thediscriminator, the correction voltage becomes zero, the tape starts toslow down and reproducer head 81 reads this block as 195 c.p.s., whichis out of the passband of the filters. For purposes of explanation theforegoing has been overly simplified because a rigorous explanation ofthe step by step action of the hunting would be too time-consuming. Itis suicient to say that if the signal is not exactly in the center ofthe band-pass of the filters, the system will hunt.

The hunting occurs only when the automatic speed control has changed thespeed of the tape so that the desired signal is in the center of theband-pass of the filters. At that time the correction voltage disappearsand the tape returns to its original speed, which again causes acorrection voltage, etc.

The fault lies with discriminator 104 shown in the block diagram ofFIGURE 6. Some compensation must be provided so that a lcorrectionvoltage will always be supplied except when the original recording is atthe proper frequency (without automatic speed correction). The carriersignal for modulator 102 is now supplied by variable frequencyoscillator 115 whose frequency is controlled by a 200 c.p.s. signalrecorded on Track No. 2 of the information tape (see FIGURE 5). Theantihunt provision can be demonstrated by going through the samesequence as explained in the paragraph on hunting. In FIGURE 5, when the195 c.p.s. block (Track No. 1) passes under head 80, a plus 5 correctivesignal is developed at discriminator 104 (135 msec. delay), the 200c.p.s. tone (Track No. 2) produces zero correction. When the 195 c.p.s.block reaches head 81, it is then corrected to 200 c.p.s. Head 80 readsthe second block (195 c.p.s.) of information as 200 c.p.s. because ofthe corrected speed but at the salme time head 80 is reading the 200c.p.s. on Track No. 2 as 205 c.p.s., which causes a five cycle change inthe insertion frequency and discriminator 104 is being supplied with afrequency that is five cycles less than the frequency being read by headS0, Track No. 1. In effect although head 80 is reading the second blockof 195 c.p.s. information as 200 c.p.s. the change in speed has caused acornpensating change in insertion frequency so that the discriminator isalways supplied with a signal at a frequency that will properly correctthe speed of the tape when it appears under head 81.

Now referring in detail to FIGURE 6 showing a readout analyzer includingcorrection apparatus, there are shown three input terminals 100, 110,and 120. Terminal 100 receives the electrical signal representative ofspeech from audio head 80 of FIGURE 5. Terminal 110 receives 200 c.p.s.signal from Track No. 2 0f FIG- URE 5. Terminal 120 receives lan inputsignal from head 81 of FIGURE 5. The input signals to tenrninals 100,110, and 120 are fed through buffer amplifiers 101, 111 and 122 and arerecived by modulators 102, 112 and 122, respectively. Modulator 102`also receives an input signal from variable crystal oscillator 115.Modulators 112 and 122 also receives input signals from fixed crystaloscillator 125. Modulator 122 supplies the signal to the filter system,modulator 102 is part of the automatic speed control apparatus, andmodulator 112 is part of the antihunt system. These three modulators areidentical.

Modulator -122 serves to convert the audio information contained in thetape to a frequency acceptable to the lter system comprised of crystalfilters 126-140 which serve as -a separation network for signalfrequencies from 100.1 kc. to 102.5 kc., respectively. -Filters 12'7-140 all operate in parallel, having a common input from modulator 122and a common output to product detector 141. Gate 142 is in thenonconductive state but when a signal is passed by filter 126 (which isthe desired signal) then the gate is open and a 100 kc. reinsertionsignal from fixed crystal oscillator is fed through gate 142 to productdetector 141. The output from product detector 141 is fed through audioamplifier 143` to output terminal 144. Audio amplifier 143 also servesto filter out the undesired portions of the 100 kc. components. Fixedfrequency oscillator 125 was designed to have a stability of one partper million per 24 hours.

As aforementioned modulator 112 receives simultaneously two inputs, onefrom buffer amplifier 111 and the other from oscillator 125. The outputfrom modulator 112 is fed to discriminator 114. It is to be noted thatdiscriminator 114 is used with variable frequency oscillator 115.Discriminator 114 gets its signal from the continuous 200 c.p.s. signalon Track No. 2 of the tape sho/wn in FIGURE 5. The signal fromdiscriminator 114 is the double sideband information from modulator 112.The output from discriminator 114 is the familiar DC shape ranging from1.87 volts to +19 volts.

Variable crystal oscillator 115 is required to vary m10 cycles which iswell within the range of tuning than can be applied to a crystal andstill insure an oscillator that exhibits crystal stability. Thefrequency may be first adjusted manually to a desired frequency and thenan automatic control of the frequency is provided by the DC voltage fromdiscriminator 114 which is applied to a voltage variable capacitorincluded in oscillator 115. This is a conventional mode of tuning anoscillator.

As previously described, modulator 102 receives simultaneously twoinputs, one from buffer amplifier 101 and the other from variablecrystal oscillator 115. The output of modulator 102 is passed throughfilter 103 to the input of discriminator 104. Discriminators 104 and 114are identical in design except that discriminator 114 gets its signalfrom the continuous 200 c.p.s. signal on Track No. 2 of the tape shownin FIGURE 5 and discriminator 104 utilizes the second harmonic of thevoice signal that appears on Track No. 1 of the tape shown in FIGURE 5.It is to be noted that the discriminator 104 receives a single sidebandsignal and the output therefrom varies from 0.78 volt to +0.88 volt DC.

Speed control component 105 consists of components 70-75 as shownconnected and operated in FIGURE 4. The output of speed controlcomponent 105 is fed through power amplier 106 to output terminal 10'7which is then connected to capstan motor which is utilized t0 controlthe speed of the tape player associated with the tape and heads and 81of FIGURE 5. It is emphasized that modulator 122 supplies the voicesignal to the filter system, modulator 102 is part of the automaticspeed control, and modulator 112 is part of the antihunt system.

The present readout analyzer is a new approach in the field of voicereadout and/or analysis. This system is capable of performing thefollowing functions: Take an individual voice that has been recorded ontape at a monotone c.p.s.i5 cycles), separate this voice into itsharmonics, then reassemble any number or all of these harmonics in theoutput until the voice has an intelligibility similar to the original.The intelligibility is very apparent to a trained listener.

The system will allow the selection of either of two voices providedthese voices are speaking in a monotone and separated by a m-inimum ofeight cycles. Assume the voices at 96 c.p.s. and 104 c.p.s., if thespeed of the tape player is reduced, the 104 c.p.s. voice will bechanged to 100 c.p.s. and the 96 c.p.s. will be changed to 92 c.p.s.,and 92 c.p.s. is outside of the passband of the filter system. Then the104 c.p.s. voice only will appear in the output. Conversely, if thespeed of the tape player is increased, the 96 c.p.s. voice will bechanged to 100 c.p.s. and the 104 c.p.s. voice will be changed to 108c.p.s., and then this time the higher frequency voice will be out of thepassband.

If the two voices described in the preceding paragraph are made to speakalternately with Voice No. 1 at 96 c.p.s. and No. 2 at 104 c,p.s., withthe tape player speed set so that these voices actually appear at theirfrequencies, it is possible to automatically pull each voiceindividually into the center Of the passband so that both voices appearin the output and both will be heard in the output speaking in the samemonotone pitch. This section demonstrates the ability of the instrumentto remove infiection.

The most important aspect of the foregoing demonstrations is the abilityfor this instrument to select any voice in a group of voices andreproduce only that voice in the output.

I claim:

1. A speech analyzer to select in intelligible form a desired voice froma multiplicity of undesired voices comprising a variable speed tapeplayer with an associated reproducer head and including said desired andundesired voices recorded upon tape, each of said voices having adifferent fundamental frequency and associated harmonics, said variablespeed tape player includes a capstan motor controlling the speed of saidtape player, first and second variable frequency crystal oscillatorshaving a preselected frequency range, first and second voltage variablecapacitors connected to said first and second variable frequencyoscillators, respectively, for varying the frequencies thereof, avariable voltage source connected to said first and second voltagevariable capacitors, said voltage source operating differentially toincrease the voltage to said first voltage variable capacitor as thevoltage to said second voltage variable capacitor decreases andconversely, a mixer receiving the outputs of said first and secondfrequency variable oscillators to provide a difference frequency signalto said capstan motor for controlling the speed thereof, a first audioamplifier receiving electrical signals representative of said voicesrecorded on said tape, a balanced modulator receiving the output signalfrom said audio amplier, a fixed crystal oscillator generating acontinuous wave signal at a preselected frequency, sa-id continuous Wavesignal also being received by said balanced modulator, said balancedmodulator creating upper and lower sidebands for all the frequenciesrecorded on said tape, a preselected number of filters connected inparallel and receiving at the input thereof said upper and lowersidebands, each of said filters having a preselected bandwidth andband-pass and each passing only said upper sideband, said preselectednumber of filters being harmonically related to each other in accordancewith a preselected fundamental voice frequency, said variable speed tapeplayer being adjusted in speed to coincide with said preselectedfundamental voice frequency to permit only said fundamental frequencyand associated harmonics of said desired voice to pass through all ofsaid preselected number of filters While partially passing saidfrequencies of said undesired voices, means to corubine the output fromsaid filters and said continuous wave signal in a product detector, anda second audio amplifier receiving the output signal from said productdetector to provide at the output thereof only said desired voice inintelligible form.

2. A speech analyzer as defined in claim 1 further includingcompensation for change of infection of said desired voice comprisingmeans to pass a preselected electrical component of said desired voicefrom said tape player at a preselected frequency bandwidth, saidelectrical component representing said change in said infiection, and adiscriminator receiving said electrical component and providing avarying direct current output in accordance with said change ininflection, said varying direct current voltage being received by saidfirst voltage variable capacitor for control thereof.

3. A speech analyzer to select in intelligible form a desired voice froma multiplicity of undesired voices, each of the voices having adifferent fundamental frequency and associated harmonics, comprising avariable speed tape player, first and second reproducer heads associatedwith said tape player, said first reproducer head positioned apreselected distance from said second reproducer head, a two track tapeassociated with said reproducer heads, the first of said tracks havingrecorded thereupon electrical signals representative of said desired andundesired voices and the second of said tracks having recorded thereupona preselected control signal of predetermined frequency for anti-huntingutilization to control said variable speed tape player, a capstan motorcontrolling the speed of said tape player in accordance with an A.C.signal, a first audio amplifier receiving electrical signals of saidvoices from said first reproducer head, a first balanced modulatorreceiving the output signal from said first audio amplifier, a firstoscillator generating a first continuons wave signal of a preselectedfrequency, said first continuous Wave signal being received by saidfirst balanced modulator, said first balanced modulator providing upperand lower sidebands of said desired and undesired electrical voicesignals, a preselected number of filters connected in parallel andreceiving at the inputs thereof said upper and lower sidebands andpassing only said upper sidebands, said preselected number of filterspassing a preselected fundamental voice frequency and harmonicallyrelated frequencies, said variable speed tape player being adjusted inspeed to coincide with said preselected fundamental voice frequency topermit only said preselected fundamental frequency and associatedharmonies of said electrical signals of said desired voice to passthrough all of said preselected number of filters while partiallypassing said frequencies of said electrical signals of said undesiredvoices, a gate normally nonconductive, receiving said first continuouswave signal for passage therethrough upon the receipt of an outputsignal from one of said preselected number of filters, a productdetector receiving simultaneously first continuous wave signal by way ofsaid gate and output signals from said preselected number of filters toprovide at the output thereof only said electrical signals of saiddesired voice, in intelligible form, a second balanced modulatorreceiving said antihunting control signal from said second track of tapeplayer, said second balanced modulator also receiving said firstcontinuo-us wave signal, a first discriminator receiving the outputsignal from said second balanced modulator to provide a first D.C.control isgnal, a voltage variable oscillator receiving said second D.C.signal to control the frequency thereof, a third balanced modulatorreceiving electrical signals of said desired and undesired voices fromsaid second reproducer head, said third balanced modulators alsoreceiving the output signal from said voltage variable oscillator, asecond discriminator receiving the output signal from said thirdbalanced modulator to provide a second D C. output signal, and a voltagevariable frequency generator receiving said second D.C. voltage andvarying the frequency thereof in accordance with said second D.C.voltage, the output of said second voltage variable frequency generatorbeing received by said capstan motor for controlling the speed thereof.

References Cited UNITED STATES PATENTS 3,079,464 2/1963 Baumel 179-1RODNEY D. BENNETT, JR., Primary Examiner C. E. WANDS, Assistant Examiner

